The present invention relates to plasma chambers and in particular to a method of measuring the build-up of film on the inner walls of such chambers.
The manufacturer of integrated circuits may employ a plasma chamber wherein semiconductor dies are treated with dopants, masked with photoresist and then etched by plasma to produce the structures needed for the particular circuit design.
Due to the nature of the plasma processing gas, a polymer film may build up on the inside wall of the plasma chamber. When this film becomes too thick, it may flake off, become trapped in the plasma and grow, eventually falling onto the dies and significantly reducing the yield. To prevent such exfoliation, the plasma chamber must be periodically cleaned, for example, by wet cleaning or oxygen ashing.
With wet cleaning, the chamber walls are cleaned with an appropriate solvent such as acetone. After this cleaning, the walls must be conditioned before the first batch of microelectronic components can be etched. Wet cleaning is a disruptive process and accordingly, it would be desired to clean the chamber as infrequently as possible.
With oxygen ashing, an oxygen plasma is introduced into the chamber to burn off the film. It can be difficult to accurately judge the length of time needed for this procedure.